1. Field of the Invention
The present invention relates generally to controller units for rear-wheel steering apparatus and, more particularly, to an electronic controller module or unit which achieves improved behavior of rear-wheel steering apparatus in vehicles, typically four-wheel steering (4WS) automotive vehicles, that are designed to steer rear wheels in association with steering of front wheels.
2. Description of the Related Art
Hitherto, four-wheel steering vehicles have been known and used popularly. The four-wheel steering vehicles are arranged to change the steering direction of the rear wheels simultaneously with steering of the front wheels in response to a manual steering maneuver of the steering wheel by a human vehicle operator or driver. During high-speed travel of the vehicle, for example, the four-wheel steering allows the vehicle to make lane changes with sufficient stability by steering the rear wheels in the same direction as the front wheels. Also, when the vehicle is traveling at low speed in a narrow space, the four-wheel steering allows the vehicle to make a small left or right turn by steering the rear wheels in an opposite direction to the front wheels. Steering direction of the rear wheels relative to a steering direction of the front wheels depends on an intended purpose of the vehicle travel (i.e., traveling speed of the vehicle, steering angle of the front wheels, etc.). Generally, every four-wheel steering vehicle includes, in addition to a front-wheel steering apparatus, a separate rear-wheel steering apparatus that steers the rear wheels, in response to a steering maneuver of the steering wheel by the vehicle operator, in association with steering of the front wheels. The rear-wheel steering apparatus constitutes an essential part of the four-wheel steering vehicles.
Basic construction and operation of the conventionally-known rear-wheel steering apparatus is outlined as follows. Two rear wheels, located on both sides of a rear body portion of the four-wheel vehicle, are connected, via knuckle arms (also called steering arms) or other connecting mechanism, to opposite ends of a single rear-wheel steering rod disposed in a widthwise direction of the vehicle. There are also provided steering-rod actuating mechanisms for moving the rear-wheel steering rod axially in a leftward or rightward direction and a drive operatively connected with the steering-rod actuating mechanisms. Namely, the rear wheels are operatively connected to the drive via the rear-wheel steering rod. When the rear-wheel steering rod is at a central neutral position with its axial midpoint substantially coinciding with the longitudinal centerline of the vehicle, the rear wheels are directed straight ahead in parallel to the longitudinal centerline of the vehicle. As the rear-wheel steering rod is moved axially leftward or rightward by the drive via the steering-rod actuating mechanisms, the two rear wheels are caused to pivot in a direction opposite to the axial movement direction of the rear-wheel steering rod.
The following paragraphs describe in greater detail an example of the conventional rear-wheel steering apparatus for accurate understanding of the construction and operation of the rear-wheel steering apparatus, with reference to the accompanying drawings. Specifically, FIGS. 16 to 22 schematically show principal parts of the rear-wheel steering apparatus proposed earlier by the assignee of the present application. More specifically, FIGS. 16 to 22 outline the rear-wheel steering apparatus disclosed in Japanese Patent Application No. HEI-11-169609, which was filed on Jun. 16, 1999 and then laid open as Japanese Patent Laid-open Publication No. 2000-79881. The present invention is directed to providing solutions to the inconveniences that would be encountered by the earlier-disclosed rear-wheel steering apparatus. The inconveniences of the earlier-disclosed rear-wheel steering apparatus will be explained below with reference FIGS. 16 to 22.
More specifically, FIG. 16 is an exploded perspective view of the earlier-disclosed rear-wheel steering apparatus. The rear-wheel steering apparatus, generally represented at reference numeral 400, includes a screw shaft 421 extending parallel to the above-mentioned rear-wheel steering rod 440. Threaded direction of the screw shaft 421 is reversed at its axial midpoint; that is, the screw shaft 421 includes left-handed and right-handed thread sections 421L and 421R. The screw shaft 421, having such left-handed and right-handed thread sections 421L and 421R, is driven by an electric motor 411, by way of a speed reduction mechanism 417, to rotate in the clockwise or counterclockwise direction. Left and right slide blocks 420L and 420R are mounted on the screw shaft 421; more specifically, lower portions of these left and right slide blocks 420L and 420R mesh with the screw shaft 421 by female-thread/male-thread engagement. By the clockwise or counterclockwise rotational movement and threaded engagement with the left and right slide blocks 420L and 420R, the screw shaft 421 functions as a drive shaft for moving the left and right slide blocks 420L and 420R in parallel to the shaft 421, as will be later described more fully with reference to FIGS. 18 and 19. Center piece 441 is fixedly mounted at the axial center of the rear-wheel steering rod 440, and the left and right slide blocks 420L and 420R are positioned adjacent opposite ends of the center piece 441 on the steering rod 440. The rear-wheel steering rod 440 are threaded through holes formed in respective upper portions of the left and right slide blocks 420L and 420R.
Clutch mechanisms 430L and 430R are attached to the left and right slide blocks 420L and 420R, respectively. Turning on the clutch mechanisms 430L and 430R causes the left and right slide blocks 420L and 420R to be operatively connected to the rear-wheel steering rod 440 that is threaded through the holes in the slide blocks 420L and 420R with the clutch mechanisms 430L and 430R interposed therebetween. Turning off the clutch mechanisms 430L and 430R, on the other hand, cancels the operative connection between the left and right slide blocks 420L and 420R and the rear-wheel steering rod 440. In this prior art rear-wheel steering apparatus, however, only either one of the slide blocks 420L and 420R is actually connected to the rear-wheel steering rod 440 with the other slide block 420R or 420L left disconnected from the steering rod 440; note that both of the left and right slide blocks 420L and 420R are never simultaneously connected to the rear-wheel steering rod 440.
The above-mentioned clutch mechanisms 430L and 430R each have an actuating lever 424L or 424R projecting outwardly from a clutch ring 432L or 432R in a direction substantially perpendicular to the axis of the rear-wheel steering rod 440. The actuating levers 424L and 424R are movable within and along a guide groove 431 as shown in FIG. 17 that is formed in a casing (not shown). Namely, horizontal groove portions of the guide groove 431 allows the actuating levers 424L and 424R to move within a predetermined horizontal range parallel to the screw shaft 421; thus, the horizontal movement of the actuating levers 424L and 424R parallel to the screw shaft 421 is limited by the length of the horizontal groove portions so that the levers 424L and 424R are horizontally movable only within the predetermined range along the screw shaft 421. Further, as clearly seen from FIG. 17, the actuating levers 424L and 424R can be moved, by activation of electrically-operated actuators 425L and 425R, along vertical groove portions of the guide groove 431 only when the left and right slide blocks 420L and 420R are at respective central or neutral positions corresponding to the axial midpoint portion of the screw shaft 421. In FIG. 17, both of the left and right slide blocks 420L and 420R are shown as resting at the uppermost positions within the corresponding vertical groove portions of the guide groove 431, in which situation the slide blocks 420L and 420R are completely prevented from moving parallel to the screw shaft 421.
The above-described construction governs the behavior of the clutch mechanisms 430L and 430R and the left and right slide blocks 420L and 420R associated with the clutch mechanisms 430L and 430R. The rear-wheel steering rod 440 is movable axially in response to the movement of the left or right slide block 420L or 420R along the screw shaft 421.
As further shown in FIG. 16, a follow-up mechanism 426 is disposed above the center piece 441, which operates to trace the axial movement of the rear-wheel steering rod 440. Steering stroke sensor 427 functions to detect a direction and amount (traveling distance) of the axial movement of the rear-wheel steering rod 440 via the follow-up mechanism 426. There is provided another follow-up mechanism 428 in association with one of the left and right slide blocks 420L and 420R (left slide block 420L in the illustrated example), which operates to trace the movement of the slide blocks 420L and 420R. Drive stroke sensor 429 functions to detect a direction and amount (traveling distance) of the axial movement of the left and right slide blocks 420L and 420R.
FIG. 16 also shows an inner stopper 491 that is located in a lower portion of the rear-wheel steering apparatus 400 and projects upward from the inner surface of the not-shown casing. This inner stopper 491 is provided for defining the neutral positions of the left and right slide blocks 420L and 420R; that is, the inner stopper 491 allows the slide blocks 420L and 420R to stop accurately at the respective neutral positions in a return-to-neutral stroke after rear-wheel steering outward movement.
FIGS. 18 and 19 are schematic top plan views of the rear-wheel steering apparatus 400, where reference numerals 444 and 445 represent left and right rear wheels. The bottom-to-top direction in these figures corresponds to a direction in which the vehicle travels straight ahead, i.e. vehicle""s straight-ahead traveling direction. Specifically, FIG. 18 shows each of the left and right rear wheels 444 and 445 having been turned left by an angle xcex8 relative to the straight-ahead traveling direction of the vehicle, while FIG. 19 shows each of the left and right rear wheels 444 and 445 being directed straight ahead or placed at a neutral position (xcex8=0) to thereby allow the vehicle to travel straight forward.
As the screw shaft 421 is rotated in one particular direction by the electric motor 411, the left and right slide blocks 420L and 420R are simultaneously moved outwardly, away from each other, by the action of the left-handed and right-handed thread sections 421L and 421R. As the screw shaft 421 is rotated in the other direction reverse to that particular direction by the motor 411, the left and right slide blocks 420L and 420R are simultaneously moved inwardly, toward each other, by the action of the left-handed and right-handed thread sections 421L and 421R. Thus, by turning on the clutch mechanism 430R before the right slide block 420R is moved outward (rightward in FIG. 18) from the central neutral position, the slide block 420R can be brought into engagement with the rear-wheel steering rod 440, so that the steering rod 440 can be moved rightward together with the rightward movement of the right slide block 420R via the screw shaft 421. By such rightward movement of the rear-wheel steering rod 440, the left and right rear wheels 444 and 445 are turned left by a steering angle xcex8 by means of the left and right knuckle arms 442 and 443, as shown in FIG. 18.
When the left and right slide blocks 420L and 420 R, having been displaced outward away from each other as shown in FIG. 18, are to be returned to their respective neutral positions so that the steering angle xcex8 of the left and right rear wheels 444 and 445 becomes zero as shown in FIG. 19, the motor 411 is rotated in the reverse direction to cause the screw shaft 421 to also rotate in the reverse direction. By the reverse rotation of the screw shaft 421, the left and right slide blocks 420L and 420 R are simultaneously moved inwardly toward each other, and thus the rear-wheel steering rod 440 engaging with the right slide block 420R is moved leftward back to its neutral position, as a result of which the left and right rear wheels 444 and 445 are returned to the straight-ahead, neutral position.
When the left and right slide blocks 420L and 420 R are simultaneously moved inwardly toward their respective neutral positions, they are ultimately brought into contact with and pressed against the left and right end surfaces, respectively, of the inner stopper 491 as denoted by arrows 498 and 499 in FIG. 19. Thus, the movement of the left and right slide blocks 420L and 420R toward each other is limited by the inner stopper 491 so that the two blocks 420L and 420 R can be mechanically stopped at their respective neutral positions. At that time, the left slide block 420L also contacts the left end surface of the center piece 441 so that further movement of the slide block 420L is prevented by the center piece 441. This way, when the rear-wheel steering rod 440 is at any other position than its neutral position due to a steering maneuver by the vehicle operator, the steering rod 440 can be accurately moved back to the neutral position by just rotating the motor 411 in the reverse direction.
Similarly to the right slide block 420R, the left slide block 420L can be brought into operative engagement with the rear-wheel steering rod 440 by turning on the associated clutch mechanism 430L, so that the left and right rear wheels 444 and 445 can be turned right and then returned to the straight-ahead, neutral position.
The prior art rear-wheel steering apparatus arranged in the above-described manner would present the following disadvantages.
When the rear wheels 444 and 445 having been turned left or right are to be returned to the straight-ahead position, the left and right slide blocks 420L and 420R are sometimes returned to their respective neutral positions at high speed, for example, in response to a quick steering maneuver by the vehicle operator. If the return-to-neutral movement of the slide blocks 420L and 420R by the screw shaft 421 is too quick, the screw shaft 421 tends to rotate excessively by an inertial rotating force of the motor 411 when the slide blocks 420L and 420R are about to abut against and thereby stop at the inner stopper 491. As a result of the excessive rotation, the screw shaft 421 would be forced excessively into the threaded holes of the slide blocks 420L and 420R as illustratively shown in FIGS. 20 to 22.
FIG. 20 shows an enlarged fragmentary view showing the right slide block 420R having been brought into contact with and pressed against the right end surface of the inner stopper 491. Namely, as the screw shaft 421 is rotated in the reverse direction (i.e., the direction denoted by arrow 451) to move back the right slide block 420R leftward (i.e., the direction denoted by arrow 499), the right slide block 420R is brought into contact with and then pressed against the right end surface of the inner stopper 491 at its neutral position. FIGS. 21 and 22 show, on a somewhat magnified scale, a portion encircled by a dot-and-dash line in FIG. 20 and more particularly explain an inconvenience of the above-discussed prior art rear-wheel steering apparatus.
More specifically, FIGS. 21 and 22 show relationship between female (or internal) thread portions 453 of the right slide block 420R and male (or external) thread portions 454 of the right-handed thread section 421R of the screw shaft 421. If the screw shaft 421 stops rotating as soon as the right slide block 420R abuts against the inner stopper 491, then there would occur no undesired distortion of the male thread portions 454, as seen in FIG. 21. However, if, due to the inertial rotating force of the motor 411, the screw shaft 421 fails to fully stop rotating immediately when the right slide block 420R abuts against the inner stopper 491, then there would occur an undesired distortion of the male thread portions 454, as seen in FIG. 22. The thus-distorted male thread portions 454 of the screw shaft 412 tends to be jammed between the female thread portions 453 of the right slide block 420R to the extent that they can not be readily disengaged from the female thread portions 453. Consequently, the rotation of the screw shaft 421 can not be resumed smoothly, or can not be resumed at all, next time the rear wheels are to be steered.
The preceding paragraph has explained the distortion and interlocking jam of the male thread portions 454 of the right-handed thread section 421R of the screw shaft 421 that would occur in returning the rear wheels to the straight-ahead, neutral position. In effect, however, a similar interlocking jam between the female and male thread portions 453 and 454 would be encountered, due to the inertial operation of the motor 411, even when the rear wheels are turned right or left to a predetermined steered position corresponding to a steering angle intended or designated by the vehicle operator, because the slide blocks 420L and 420R moving outward by the screw shaft 421 are stopped mechanically by being engaged by outer stoppers that project from the inner surface of the not-shown casing similarly to the above-mentioned inner stopper 491.
To avoid the interlocking jam between the female and male thread portions 453 and 454 when the slide blocks 420L and 420R are moved to left and right outward positions corresponding to a designated steering angle of the rear wheels or returned to the neutral positions after the outward movement, it is necessary that the screw shaft 421 with the male thread portions 454 have a sufficiently great outer diameter and the left and right slide blocks 420L and 420R have a sufficiently great inner diameter and that the male thread portions 454 and female thread portions 453 have increased rigidity. However, the increases in the outer diameter of the screw shaft 421, inner diameter of the slide blocks 420L and 420R and rigidity of the thread portions 453 and 454 would unavoidably lead to an increased overall size and weight of the rear-wheel steering apparatus 400. The rear-wheel steering apparatus 400 of the increased overall size and weight would require a large-size and high-power electric motor 411 and high-power drive circuit operating with great electric current, thereby unavoidably resulting in increased manufacturing costs of the apparatus.
Inconveniences due to the above-mentioned interlocking jam in threaded engagement between the male and female thread portions would also be encountered in other types of rear-wheel steering apparatus than the above-discussed type as long as they are provided with sliding members movable leftward and rightward via a screw shaft mechanism.
In view of the foregoing problems, it is an object of the present invention to provide an improved controller unit for a rear-wheel steering apparatus of a four-wheel steering vehicle which, by improved electric and electronic control, can effectively avoid an unwanted interlocking jam between thread portions of sliding members and thread portions of a screw shaft mechanism which would occur when the sliding members are moved by the screw shaft mechanism to left and right outward positions corresponding to a designated steering angle or when the sliding members are returned from the left and right outward positions to their respective central neutral positions, without a need for increasing the overall size and weight of the rear-wheel steering apparatus.
Controller unit of the present invention is designed to control operation of a rear-wheel steering apparatus typically provided in a four-wheel steering automotive vehicle. The rear-wheel steering apparatus, to which the present invention is applied, comprises: a motor for driving rear-wheel steering; a rotational-force transmitting member, such as in the form of an outer rotational-force transmitting cylinder, that is rotatable in accordance with rotation of the motor; a pair of sliding members, such as in the form of left and right sliding cylinders, movable from respective central neutral positions, away from each other, to respective outward positions corresponding to a designated steering angle of rear wheels and movable from the outward positions, toward each other, back to the neutral positions; a threaded engagement section having oppositely-threaded portions and operatively connecting between the rotational-force transmitting member and the sliding members through threaded engagement in such a manner that the sliding members are caused to move away from or toward each other in response to rotation of the rotational-force transmitting member by the motor; a clutch mechanism for operatively connecting either one of the sliding members with a steering rod provided between the rear wheels; and a first stopper for defining the neutral positions of the sliding members and limiting respective inward movement of the sliding members. Here, with the one of the sliding members operatively connected with the steering rod via the clutch mechanism, the rear wheels are turned by a designated steering angle by moving the sliding members outwardly away from each other through rotation of the motor in a first (i.e., forward) direction and are returned to a straight-ahead position by moving the sliding members inwardly toward each other through rotation of the motor in a second (i.e., reverse) direction. In the return-to-neutral stroke of the sliding members, the sliding members are stopped at the respective neutral positions mechanically by the first stopper.
The controller unit according to one aspect of the present invention is based on improved electronic circuitry which comprises a target-steering-angle setting section, an actual-steering-angle measurement device including a steering stroke sensor and an actual-steering-angle measurement section, a steering-angle adjustment section, and a motor-deceleration-amount calculation section. The target-steering-angle setting section sets a target steering angle of the rear wheels to provide an electric signal indicative of the target steering angle. The actual-steering-angle measurement section measures an actual steering angle of the rear wheels to provide a signal indicative of the measured actual steering angle. On the basis of the target steering angle set by the target-steering-angle setting section and the actual steering angle measured by the steering-angle adjustment section, the steering-angle adjustment section generates a motor drive signal to control the rotation of the motor in such a manner that a difference or offset between the target steering angle and the actual steering angle becomes zero, and then the steering-angle adjustment section supplies the motor drive signal to the motor so that, through the rotation of the motor based on the motor drive signal, the sliding members are moved to the outward positions when the rear wheels are to be turned by the designated steering angle or the sliding members are returned to the neutral positions when the rear wheels are to be turned back to the straight-ahead position. On condition that the motor is in a predetermined driving state, the motor-deceleration-amount calculation section is activated to calculate an actual rotating speed of the motor on the basis of the actual steering angle of the rear wheels and compares the calculated actual rotating speed of the motor with a prestored reference rotation speed. The motor-deceleration-amount calculation section operates to calculate a motor-rotation deceleration amount when the compared result indicates that the calculated actual rotating speed is higher than the reference rotation speed. Further, in the rear-wheel steering controller unit, the steering-angle adjustment section includes a subtracter for subtracting the motor-rotation deceleration amount from the motor drive signal.
When the sliding members are to be returned to their respective neutral positions after having moved the rear wheels leftward or rightward, the sliding members are compulsorily stopped mechanically by the first stopper formed on a casing, during which time the controller unit of the present invention compares the calculated actual rotating speed of the motor with the prestored reference rotation speed and adjusts the motor drive signal, in accordance with the compared result, to slow down the motor rotation, in order to prevent an interlocking jam between the sliding members and the rotational-force transmitting member from occurring due to the inertia of the motor.
In the inventive rear-wheel steering controller unit, the above-mentioned predetermined driving state of the motor is a state in which the motor is rotating to return the sliding members to the neutral positions and the sliding members have entered a predetermined decelerating region close to the respective neutral positions. Thus, when the motor is in the predetermined driving state, the rotation of the motor is decelerated, in accordance with the motor-rotation deceleration amount calculated by the motor-deceleration-amount calculation section, immediately before the sliding members are stopped at the neutral positions.
The rear-wheel steering apparatus may include second stoppers for defining respective outermost positions of the sliding members to limit the outward movement of the sliding members. In this case, the predetermined driving state of the motor is a state in which the motor is rotating to move the sliding members to the outward positions and the sliding members have entered a predetermined decelerating region close to the outermost positions. Thus, when the motor is in the predetermined driving state, the rotation of the motor is decelerated, in accordance with the motor-rotation deceleration amount calculated by the motor-deceleration-amount calculation section, immediately before the sliding members are stopped at the outermost positions.
In a preferred implementation of the present invention, the motor-deceleration-amount calculation section includes an actual steering angle differentiator for calculating the actual rotating speed of the motor, and a motor rotation speed table or map for selectively providing a value of the reference rotation speed corresponding to the actual steering angle of the rear wheels measured by the actual-steering-angle measurement section.
According to a second aspect of the present invention, the rear-wheel steering controller unit is characterized by comprising, in addition to the above-mentioned target-steering-angle setting section, actual-steering-angle measurement device, and steering-angle adjustment section, a motor drive limiting section for, on condition that the motor is in a predetermined driving state and the actual steering angle of the rear wheels has entered a predetermined decelerating region, being activated to generate a motor drive limiting signal for limiting the rotation of the motor. In this case, the steering-angle adjustment section adjusts the motor drive signal in accordance with the motor drive limiting signal to thereby decelerate or slow down the rotation of the motor.
In a preferred implementation of the present invention, the motor drive limiting signal is given to a motor drive control section provided in the steering-angle adjustment section for generating a PWM-controlled signal, and wherein generation of the PWM-controlled signal by the motor drive control section is stopped in accordance with the motor drive limiting signal.
Preferably, the motor drive limiting signal is given to a motor-driving bridge circuit provided in the steering-angle adjustment section for generating the motor drive signal. The electrical connecting state within the motor-driving bridge circuit may be varied, in accordance with the motor drive limiting signal, to thereby form a short-circuiting circuit for the motor such that the rotation of the motor is stopped compulsorily through self-power-generating operation of the motor.
Preferably, respective on/off states of four switch elements constituting the motor-driving bridge circuit are controlled in accordance with the motor drive limiting signal, to thereby form the short-circuiting circuit for the motor.
The motor-driving bridge circuit may include a relay section to permit formation of the short-circuiting circuit, and the short-circuiting circuit is provided by controlling the relay section in accordance with the motor drive limiting signal.
Preferably, the above-mentioned predetermined driving state of the motor is a state where the motor is rotating to return the sliding members to the neutral positions and the sliding members have entered a predetermined decelerating region close to the neutral positions, and wherein when the motor is in the predetermined driving state, the motor drive limiting section limits the rotation of the motor immediately before the sliding members are stopped at the neutral positions.
The rear-wheel steering apparatus may further include second stoppers for defining respective outermost positions of the sliding members to limit outward movement of the sliding members, and the predetermined driving state of the motor is a state where the motor is rotating to move the sliding members to the outward positions and the sliding members have entered a predetermined decelerating region close to the outermost positions. Thus, when the motor is in the predetermined driving state, the motor drive limiting section limits the rotation of the motor immediately before the sliding members are stopped at the outermost positions.